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Where are you getting your information? Are you saying you need a diode across the AC relay coils but not DC relay coils? You need them for DC relays because the coils are inductive and when you de-energize them (stop the current from flowing through them) it creates damaging voltage spikes. The diode is used to supress this voltage spike.aljamri said:and why do we need the free-wheeling-diode in AC? !
Yes, it's a single heavy loop of wire.dknguyen said:What is a shorted turn? You mean just a loop of wire? ?? ??
An inductor's transfer function is jwc, so wouldn't that account for the 90 degree phase shift?
V=IZ, Z = jwL
So that would mean that the voltage across an inductor leads the current in the inductor by 90 degrees in the complex plane.
While relays are usually taught and more easily understood as DC devices, in
practice AC relays are commonly used. These relays typically employ shaded
poles, similar to the shaded pole induction motor. A shaded pole is a coil loop
which is not separately excited by the relay source; it is excited by the flux on
the main relay coil. This coil then produces an opposing current, flux and
voltage (Faraday’s and Lenz’s law) which holds the contact during zero voltage
intervals on the main coil. Failure of the shaded pole leads to “relay
chatter”— a 120 Hz clicking that occurs every time the main relay voltage
crosses the zero point.
dknguyen said:I was getting confused about the previous description of a shaded loop being a short-circuit (I was imagining the loop as completing a short-circuit whicih didn't make sense).
................................................................................Ron H said:....
I don't understand how the current is shifted 90 degrees. Perhaps someone else can explain this.
But the magnetic field in the relay's solenoid is in phase with the current in the "primary". Since the shorted turn is in that same field , won't the current in the shorted turn be in phase with the primary?hawk2eye said:................................................................................
Ron H.
relay coil AC current 90_degrees, wrt Line Voltage which produces the short circuit current
-AC Line Voltage generates current I1 through the relay inductance, L
I1 = Vac/wL at 90_degrees relative to Voltage
-AC Line Voltage causes magnetic field to operate relay
-AC Line Voltage through ' transformer", coil inductance as primary
sc_turn as secondary, causes current I2 through sc_turn
0_degrees relative to Voltage
hawk2eye
eblc1388 said:Now you are confused.
You've imaged correctly. It is physically a completed loop joined end to end to form a perfect short-circuited arrangement. It make perfect sense.
But the magnetic field in the relay's solenoid is in phase with the current in the "primary". Since the shorted turn is in that same field , won't the current in the shorted turn be in phase with the primary?
The purpose of the shaded pole is to create a hysteresis loop so the magnetic field switches so fast that the magnetic armature does not have a chance to open before it's pulled back in. If the shorted ring opens up, the relay or contactor will buzz like hair clippers that are adjusted too tight. If you look at the face of the pole piece, you'll see that only part of the armature has the shorted turn on.
Kinarfi
the relay will buzz.